Since the discovery of high oxide superconducting materials having critical temperatures that exceed the temperature of liquid nitrogen, there has been a concerted effort to utilize these materials for various applications. Although many applications are aimed at replacing conventional superconductors in wires and electronic devices, new applications using bulk materials also have been proposed. These applications include, for example, use in energy storage devices such as flywheels, use in current leads for superconducting magnets, and in magnetic bearings, bulk magnets, and magnetic resonance imaging machines (MRI).
For applications such as those noted in the proceeding paragraph, high temperature superconducting materials with large critical current density (J.sub.c) are required. One such high temperature superconducting material is a composite oxide of RE, Ba and Cu.sub.1 (ReBCO) and in particular, REBa.sub.2 CU.sub.3 O.sub.x (wherein RE represents at least one of the following rare earth elements: Y, La, Sm, Nd, Eu, Gd, Dy, Ho, Er, Tm, Yb, or Lu).
Current materials research aimed at fabricating high temperature superconducting ceramics in conductor configurations for bulk, practical applications, is largely focused on powder-in-tube methods. Such methods have proven quite successful for the Bi--(Pb)--Sr--Ca--Cu--O (BSCCO) family of superconductors due to their unique mica-like mechanical deformation characteristics. In high magnetic fields, however, this family of superconductors is generally limited to applications below 30.degree. K. In the ReBCO Tl--(Pb, Bi)--Sr--(Ba)--Ca--Cu--O and Hg--(Pb)--Sr--(Ba)--Ca--Cu--O families of superconductors, some of the compounds have much higher intrinsic limits and can be used at higher temperatures.
It has been demonstrated that these superconductors possess high critical current densities (J.sub.c) at high temperatures when fabricated as single crystals or in essentially single-crystal form as epitaxial films on single crystal substrates such as SrTiO.sub.3 and LaAlO.sub.3. These superconductors have so far been intractable to conventional ceramics and materials processing techniques to form long lengths of conductor with a J.sub.c comparable to epitaxial films. This is primarily because of the "weak-link" effect.
It has been demonstrated that with ReBCO, biaxial texture is necessary to obtain high transport critical current densities. High J.sub.c 's have been reported in polycrystalline ReBCO in thin films deposited in special substrates on which a biaxially textured non-superconducting oxide buffer layer is first deposited using ion-beam assisted deposition (IBAD) techniques. However, IBAD is a slow, expensive process, and difficult to scale up for production of lengths adequate for many applications.
High J.sub.c 's have also been reported in polycrystalline ReBCO melt-processed bulk material which contains primarily small angle grain boundaries. Melt processing is also considered too slow for production of practical lengths.
Thin-film materials having perovskite-like structures are important in superconductivity, ferroelectrics, and electro-optics. Many applications using these materials require, or would be significantly improved by single crystal, c-axis oriented perovskite-like films grown on single-crystal or highly aligned metal or metal-coated substrates. For instance, Y--Ba.sub.2 --Cu.sub.3 --O.sub.-- (YBCO) is an important superconducting material for the development of superconducting current leads, transmission lines, motor and magnetic windings, and other electrical conductor applications. When cooled below their transition temperature, superconducting materials have no electrical resistance and carry electrical current without heating up.
One technique for fabricating a superconducting wire or tape is to deposit a YBCO film on a metallic substrate. Superconducting YBCO has been deposited on polycrystalline metals in which the YBCO is c-axis oriented, but not aligned in-plane. To carry high electrical currents and remain superconducting, however, the YBCO films must be biaxially textured, preferably c-axis oriented, with effectively no large-angle grain boundaries, since such grain boundaries are detrimental to the current-carrying capability of the material. YBCO films deposited on polycrystalline metal substrates do not generally meet this criterion.
Many electronic, magnetic, or superconductor device applications require control of the grain boundary character of the device materials. For example, grain boundary character is very important in high temperature superconductors. The effects of grain boundary characteristics on current transmission have been clearly demonstrated for certain materials, for example, the material known as Y123. See Dimos, et al. (1988) Phys. Rev. Lett. 61:219; and Dimos, et al. (1990) Phys. Rev. Lett. 41:4038.
For clean, stoichiometric boundaries, the grain boundary critical current (J.sub.c (gb)) appears to be determined primarily by grain boundary misorientation. The dependence of J.sub.c (gb) on misorientation angle for Y123 has been determined by Dimos et al., supra, for grain boundary types which can be formed in epitaxial films on bicrystal substrates. These include [001] tilt, [100] tilt, and [100] twist boundaries. In each case, however, high angle boundaries were found to be weak-linked.
Recently, the Dimos work has been extended to artificially fabricated [001] tilt bicrystals in Tl.sub.2 Ba2CaCu.sub.2 O.sub.8 (A. H. Cardona, et al., Appl. Phys. Lett., 62 (4), 411, 1993)), Ndl.sub.85 Ce.sub.0 15 CuO.sub.4, Tl.sub.2 Ba.sub.2 Ca.sub.2 Cu.sub.3 O.sub.x (M. Kawasaki, et al., Appl. Phys. Lett., 62 (4), 417 (1993)), and TlBa.sub.2 Ca.sub.2 Cu.sub.2 O.sub.x (T. Nabatame, et al., Appl. Phys. Lett. 65 (6), 776 (1994)). In each of these cases, it was found that, as in the case of Y123,J.sub.c depends strongly on grain boundary misorientation angle. Although no measurements have been made on the material known as Bi-2223, data on current transmission across artificially fabricated grain boundaries in the material termed Bi-2212 indicate that most large angle [001] tilt (M. Kawasaki, et al., Appl. Phys. Lett., 62 (4), 417 (1993)) and twist boundaries are weak links, with the exception of some coincident site lattice (CSL) related boundaries (N. Tomita, et al., Jpn. J. Appl. Phys., 29 (1990) L30; N. Tomita, et al., Jpn. J. Appl. Phys., 31, L942 (1992); J. L. Wang, et al., Physica C, 230, 189 (1994)). It is likely that the variation in J.sub.c with grain boundary misorientation in materials Bi-2212 and Bi-2223 is similar to that observed in the well characterized cases of Y123 and Tl-based superconductors.
Hence, in order to fabricate high temperature superconductors with very critical current densities, it will be necessary to biaxially align all the grains. This has been shown to result in significant improvement in the superconducting properties of YBCO films (Y. lijima, et al., Appl. Phys., 74, 1905 (1993); R. P. Reade et. al., Appl. Phys. Lett., 61, 2231 (1992); X. D. Wu, et al., Appl. Phys. Lett., 65, 1961 (1994).
In U.S. patent application Ser. No. 08/419,583, now issued as U.S. Pat. No. 5,741,377, which is incorporated by reference herein, we disclosed biaxially textured articles which comprise a rolled and annealed metal surface having a face-centered cubic (FCC), body centered cubic (BCC) or hexagonal close-packed (HCP) crystalline structure and a biaxially textured layer of an electronic device on a surface thereof. For example Cu with a sharp cube texture was obtained by deforming Cu by large amounts (90%) followed by recrystallization. However, this was possible only in high purity Cu, Ni or Al. Even small amounts of impurity elements (e.g., 0.0025% P, 0.3% Sb, 0.18% Cd, 047% As, 1% Sn, 0.5% Be, or the like) can severely suppress the formation of the cube texture.
When a polycrystalline material is subjected to the rolling process, plastic flow causes reorientation of the lattice of individual grains of the polycrystalline material and tends to develop a texture or preferred orientation of the lattice in the grains. The progress of reorientation is gradual; the orientation change proceeds as plastic flow continues, until a texture is reached that is stable against indefinite continued flow of a given type. The nature of the stable deformation texture and the manner in which it is approached is characteristic of the material and of the nature of the flow throughout the deformation process (i.e., the magnitude of the three principal strains at all points within the specimen and at successive times during the process). The texture development is strongly influenced by temperature, particularly if the temperature of deformation is high enough for recrystallization to take place. Other effects of temperature include variation of the stacking fault energy and hence the operative deformation mechanisms. In general, plastic strains near the surface of a rolled specimen may differ from those in the interior and may produce textures that vary with depth below the surface. Hence specific rolling procedures, which are described herein below, are used to ensure reasonably consistent textures through the thickness of the work piece.
While forward rolling alone may result in homogeneous texture through the thickness of the sheet, we have found that reverse rolling (rolling direction is reversed after each pass) produces much better results in most materials. In most of what is described below, reverse rolling is preferred over forward rolling. The rolling speed and reduction per pass are also important parameters. While rolling speed may be important in the texture development, its effect is not dominating. In general, higher rolling speeds are desirable for economical purposes. Reduction per pass during rolling is also important for texture development. Generally, less than 30% reduction per pass is desirable, although in some cases higher reductions per pass may also be required. The lubrication employed during rolling is also an important variable. Depending on the texture desired, either no lubricant or some lubricant like light mineral oil, heavy mineral oil, kerosene, etc. are employed to ensure homogeneous texture development. Grain size of the starting material and prior heat treatments and deformation history is also important in determining the texture development. In general, prior to rolling, a fine grain size is desired and the initial heat treatments and deformations are designed to give a random texture in the starting material.
The development of an annealing texture involves several fundamental mechanisms. An annealing texture may develop from recovery without recrystallization (in which case it would be expected to duplicate the texture present before annealing), from primary recrystallization, or from grain growth subsequent to recrystallization. Grain size distribution can remain normal throughout the process, or a few grains may grow very large while the rest remain approximately unchanged until devoured by the large ones. The latter type of grain growth, referred to as "secondary recrystallization" or "discontinuous", is generally considered to be abnormal.
It is known that the critical current density through a grain boundary may be reduced significantly for misorientation angles greater than 5.degree.-10.degree.. It is thus desirable to obtain superconducting deposits in which the number of grain boundaries with misorientation angles greater than 5.degree.-10.degree. is minimized. For conductors in which the superconducting deposit is epitaxial with an underlying metallic or oxide buffer layer or substrate, it is desirable to minimize the number of grain boundaries with misorientations greater than 5.degree.-10.degree.. This is accomplished if the texture of the substrate is so sharp that grain orientations vary by no more than 5.degree.-10.degree.. Useful superconducting layers may be obtained using substrates with larger spread in grain orientation, but the better the substrate texture, the better the properties of the superconductor deposit are expected to be.
In a cube texture, the cube plane, (100), lies parallel to the plane of the sheet and a cube edge, [001], is parallel to the rolling direction, i.e. {100}&lt;001&gt;. This texture resembles a single crystal with subgrains, but may contain a minor amount of material in twin relation to the principal orientation. A fully developed cube texture, as described herein, has been developed with biaxial alignment with x-ray diffraction peak width of 8-30.degree. full width at half maximum.
A method to produce biaxially textured substrates has been proposed to produce a sharp cube texture on FCC metals like copper (Cu) or nickel (Ni). In this process, the metal is first thermomechanically textured, followed by epitaxial growth of additional metal or ceramic layers. Epitaxial YBCO films grown on such substrates resulted in high critical current J.sub.c. However, the nature of the substrate was inadequate for being useful in many applications such as superconductors.
The preferred substrate used in the prior process comprised high purity Ni. Since Ni is ferromagnetic, the substrate as a whole is magnetic, which can cause significant problems in practical applications involving superconductors. In addition, the thermal expansion of the Ni substrate used in the process dominates that of most ceramic layers desired for practical applications. This mismatch can result in cracking which can limit its usefulness. Closer matching of the lattice parameter of the Ni and the ceramic layer material can prevent cracking and other stress related defects, as well as effects (e.g. delamination), in the ceramic films. However, this would require modification of the lattice parameter to be closer to that of the ceramic layers. Lastly, annealed, well textured, high purity Ni is very weak. For most applications involving superconducting tapes or wires, the conductor should be able withstand certain Lorentz forces during operation. Significant improvements in strain tolerance, handleability and strength of the textured substrates are required.
Thus, there is a need to provide new biaxially textured substrates for use in many applications, such as superconductors, and methods of making such substrates.
For further background information, refer to the following publications:
1. K. Sato, et al., "High-J.sub.c Silver-Sheathed Bi-Based Superconducting Wires", JEFE Transactions on Magnetics, 27 (1991) 1231. PA1 2. K. Heine, et al., "High-Field Critical Current Densities in Bl.sub.2 Sr.sub.2 Ca.sub.1 Cu.sub.2 O.sub.8 +x/Ag Wires", Applied Physics Letters, 55 (1991) 2441. PA1 3. R. Flukiger, et al., "High Critical Current Densities in Bi(2223)/Ag Tapes", Superconductor Science & Technology, 5 (1992) S61. PA1 4. D. Dijkkamp, et al., "Preparation of Y--Ba--Cu Oxide Superconducting Thin Films Using Pulsed Laser Evaporation from High Te Bulk Material", Applied Physics Letters, 51, 619 (1987). PA1 5. S. Mahajan, et al., "Effects of Target and Template Layer on the Properties of Highly Crystalline Superconducting a-Axis Films of YBa.sub.2 --Cu.sub.3 --O.sub.7 by DC-Sputtering", Physica C., 213, 445 (1993). PA1 6. A. Inam, et al., "A-axis Oriented Epitaxial YBa.sub.2 --Cu.sub.3 --O.sub.7 --PrBa.sub.2 Cu.sub.3 O.sub.7 Heterostructures", Applied Physics Letters, 57, 2484 (1990). PA1 7. R. E. Russo, et al., "Metal Buffer Layers and Y--Ba--Cu--O Thin Films on Pt and Stainless Steel Using Pulsed Laser Deposition", Journal of Applied Physics, 68, 1354 (1990). PA1 8. E. Narumi, et al., "Superconducting YBa.sub.2 Cu.sub.3 O.sub.-- Films on Metallic Substrates Using In Situ Laser Deposition", Applied Physics Letters, 56, 2684 (1990). PA1 9. J. D. Budai, et al. "In-Plane Epitaxial Alignment of YBa2--Cu.sub.3 --O.sub.7 Films Grown on Silver Crystals and Buffer Layers", Applied Physics Letters, 62, 1836 (1993). PA1 10. T. J. Doi, et al., "A New Type of Superconducting Wire; Biaxially Oriented Tl1(Ba.sub.0.8 S.sub.r0.2).sub.2 Ca.sub.2 Cu.sub.3 O.sub.7 on [100]&lt;100&gt; Texture Silver Tape", Proceedings of 7th International Symposium on Superconductivity, Fukuoka, Japan, Nov. 8-11, 1994. PA1 11. D. Forbes, Executive Editor "Hitachi Reports 1-meter Tl-1 223 Tape Made by Spray Pyrolysis", Superconductor Week, Vol. 9, No. 8, Mar. 6, 1995. PA1 12. Recrystallization Grain Growth and Textures, Papers presented at a seminar of the American Society for Metals, Oct. 16 and 17, 1965, American Society for Metals, Metals Park, Ohio. PA1 13. A. Goyal et al., "High Critical Current Density Superconducting Tapes by Epitaxial of YBa.sub.2 Cu.sub.3 Ox Thick Films on Biaxially Textured Metal Substrates", Appl. Phys. Lett., 69, 1795 (1996). PA1 14. D. P. Norton et al., "Epitaxial YBa.sub.2 Cu.sub.3 O.sub.x on Biaxially Textured Biaxially Textured (001) Ni: An Approach to High Critical Current Density Superconducting Tape", Science, 274, 755 (1996). PA1 15. M. Paranthaman et al., "Growth of Biaxially Textured Buffered Layers on Rolled Ni Substrates by Electron Beam Evaporation" Physica C., 275, 266(1997).